The invention relates to a method of treating a silicon wafer for forming a thin, buried layer of insulating material close to the surface of the wafer, and specifically to a process wherein a first ion species is implanted into the silicon to create crystal defects therein, and a second ion species is then diffused into the silicon to create an insulating layer.
Silicon-on-insulator (SOI) materials have been found to be particularly useful in ultra large scale integration (ULSI) complementary metal oxide semiconductor (CMOS) applications. One technique for making SOI materials is to implant nitrogen or oxygen ions into a silicon wafer. The implantation provides a layer of mixed phase material beneath the upper surface of the silicon, which layer contains varying degrees of crystal defects therein. Such implantation is normally conducted with relatively high energy implantation, in the range of 150 KeV to 1 MeV, producing ion doses on the order of 10.sup.18 cm.sup.-2. Such activities were reported by Lam and Pinizzotto, Journal of Crystal Growth, North-Holland Publishing Company, 1983. Lam et al., and others have described a number of techniques for treating the defective layer formed by implantation, which usually involves high-temperature annealing to provide some cure to the defective region and to cause a reaction to form the insulating layer, thereby leaving a single crystal layer above the insulating layer. Traditionally, an epitaxial layer is then grown on the silicon in order to provide multiple layers for the construction of semiconductor structures. In the course of implantation, the silicon wafer is typically heated by both the implantation beam and external sources.
Fabrication of thin SOI films by low-energy oxygen ion implantation with relatively low doses of ions have been described by Ishikawa et al., Japanese Journal of Applied Physics, Vol. 30, No. 10, October, 1991, pages 2427-2431. In that report, oxygen ions were implanted at an energy of 15-30 KeV, and a resulting dose of approximately 10.sup.17 ion/cm.sup.2. This resulted, in one instance, in an amorphous top Si layer of 48 nm and an SiO.sub.2 layer thereunder of 61 nm. The interface between the top layer and the amorphous layer was 3.5 nm.
Ibrahim, Materials Chemistry and Physics, 31 (1992) (285-300), "Synthesis of Buried Insulating Layers in Silicon by Ion Implantation" suggest at 294, that simultaneous implantation using NO.sup.+ or N.sub.2 O.sup.+ molecules may result in the formation of buffed silicon-oxy-nitride layers. However, at 295, they discount this technique, stating that simultaneous implanting leaves an oxy-nitride layer too near the projected range, and that excess oxygen will accumulate at the interface of the silicon and oxy-nitride matrix, causing a deterioration, with depth, in the quality of the top single-crystal silicon.
Our previous U.S. Pat. No. 5,436,175, granted Jul. 25, 1995, discloses a method of forming shallow SIMOX (Separation by IMplantation of OXygen) substrates by implanting molecular oxygen ions into a substrate, in order to form a silicon dioxide (SiO.sub.2) insulating layer. The process utilized a molecular ion implantation beam to implant a dose of molecular ions, either oxygen or nitrogen, having a dose of between 0.6.multidot.10.sup.17 and 2.5.multidot.10.sup.17 molecular ions per cm.sup.2, and an energy range of between 60 KeV and 90 KeV. The substrate is annealed, which causes the implanted insulating element to react with the silicon, forming either SiO.sub.2 or Si.sub.3 N.sub.4. Alternatively, both oxygen and nitrogen may be implanted, with the substrates having the implanted materials then annealed to form a silicon-oxy-nitride (Si.sub.x N.sub.y O.sub.z) insulating layer.
A drawback with the known prior art methods of making silicon-on-insulator products is that the wafer must be annealed for a rather lengthy period of time in order to cure the defects formed therein during the implantation process. This results from a variety of factors, including the relatively high energies that are used to implant an insulating substance, the depth of the insulating layer, and the requirement to diffuse other substances through the non-insulating layer to the defect or insulating layer.